1. Field of the Invention
The present invention relates to a toner for use in electrophotography, electrostatic printing, or a toner jet recording method, and a two-component developer comprising the toner.
2. Description of the Related Art
The following methods have been generally used in recent years in proposed full-color copying machines and full-color printers. One method is a method for forming a full-color image, the method including: using four photosensitive members and a belt-shaped transfer body; developing an electrostatic charge image formed on each photosensitive member with a cyan toner, a magenta toner, a yellow toner, and a black toner severally; and sequentially transferring a toner image onto the photosensitive member while transporting a transfer material to a position between the photosensitive member and the belt-shaped transfer body. Another method includes: winding a transfer material on the surface of a transfer body that is opposed to a photosensitive member by an electrostatic force or a mechanical action such as that of a gripper; and performing a step of developing and a step of transferring four times to obtain a full-color image.
Toners to be loaded into those full-color copying machines and full-color printers require an improvement in color reproducibility and sufficient color mixing of the respective toners during a step of heat and pressure fixing without impairment of transparency of an overhead projector (OHP) image.
Moreover, a toner has been recently required to have functions that allow adaptation to high speed processing and to on-demand printing. In addition, the toner is required to achieve improved better low temperature fixability, expansion of a non-offset area, and control of a gloss.
In the conventional method, in order to achieve the above-described objects, a fixing member has been generally used with applying silicone oil to the surface of a fixing member. However, the conventional method involves contamination in a machine due to vaporization of the silicone oil and the difficulty in achieving evenness of application. Thus, enhanced releasability has been recently imparted to a toner.
JP 08-314300 A and JP 08-050368 A each propose a toner produced by encapsulating wax in a toner particle via suspension polymerization, and an image forming method in which the toner is used so that no silicone oil is used.
Although each of those toners suppresses an oil streak on a fixed image, each of those toners requires encapsulation of a large amount of wax in a toner particle, and uses a binder mainly composed of a styrene-acrylic resin. As a result, irregularities on the surface of the fixed image may become a problem. Therefore, a further improvement in OHP permeability has been demanded.
Because image recorded articles made by those toners have low glosses, there is a merit that a satisfactory image with no sense of incongruity can be obtained in a graphic image in which a graph and a character portion mix. However, in a pictorial image, the toner is not sufficiently melted when the toner is fixed, and thus there is a demerit that the color-mixing property of a secondary color is low to result in a narrow color reproduction range.
In view of the above, a toner has been awaited, which is excellent in low temperature fixability, which achieves gloss control, which is excellent in color-mixing property, which provides a wide color reproduction range, and which is excellent in OHP permeability when a heat and pressure fixing means is used in which no oil is used or oil usage is reduced. As a method of achieving such a toner, an attempt has been made to employ polyester having a high sharp melt property as a main binder.
Furthermore, from the viewpoint of realizing print on demand, the need for dealing with various materials including cardboard and coat paper arises, so that a transfer method using an intermediate transfer body has been becoming increasingly effective. A toner is developed onto a photosensitive member, and is then temporarily transferred onto an intermediate transfer body. After that, the toner is transferred onto a transfer material. Therefore, a toner having higher transfer efficiency is desired.
Known as such a toner is a toner which is excellent in developability and transferability, which provides satisfactory offset resistance in an oilless fixing system, and which is excellent in OHP permeability.
JP 11-044969 A proposes a sphered toner produced by: using a linear polyester resin or a non-linear polyester resin; dispersing the polyester resin, a colorant, and a releasing agent in an organic solvent in which the resin dissolves; dispersing the resultant liquid in an aqueous medium for granulation; and removing the organic solvent under reduced pressure in this state. The toner obtained exhibits extremely high transferability, and is excellent in hot offset resistance.
However, the toner involves the difficulty in adjusting particle diameters of toner particles of 5 μm or less, and in the toner, improved low temperature fixability is required for further high speed processing.
JP 07-181732 A proposes, as a method of producing such a toner, a method in which a toner comprising a colorant and a releasing agent, the toner is sphered with a mechanical impact force to enhance transfer efficiency. Examples of a known apparatus for speroidization with a mechanical impact force include Hybridization System manufactured by Nara Machinery Co., Ltd., Mechanofusion System manufactured by Hosokawa Micron Corp., and Cryptron System manufactured by Kawasaki Heavy Industries, Ltd.
However, each of those systems is a system that applies a mechanical impact force during pulverization of a toner, although those systems differ from one another in degree of pulverization. Therefore, exudation of a releasing agent due to the appearance of a new surface simultaneously with sphering tends to occur, so that developability may decrease. In particular, in the case where the releasing agent is poorly dispersed, the exudation of the releasing agent becomes remarkable.
In addition, a reduction in toner particle diameter has been carried out to improve dot reproducibility and fine line reproducibility. However, in a toner which provides low temperature fixability and hot offset resistance, and which comprises a polyester resin and a releasing agent as described above to obtain a high gloss, a reduction in toner particle diameter results in an abrupt increase in toner specific surface area. Therefore, it has been difficult to prevent both the exudation of the releasing agent upon heat and pressure fixing and the exudation of the releasing agent due to the stress applied to the toner upon impartment of frictional electrification in development.
JP 2000-003075 A proposes sphering and uniformizing shapes of particles in a developer (a toner or a toner and a magnetic carrier) obtained by a kneading-pulverization method to thereby uniformize charge.
Sphering of the toner mitigates scattering and improves transferability. However, a toner sphered through hot air treatment makes it difficult to control the state of existence of a releasing agent (hereinafter, referred to as “releasing-agent existence state”) near the toner surface, thereby making it difficult to satisfy low temperature fixability and developability at the same time.